Assertion : In muscle contraction, length of both A-bands and I-bands decrease.
Reason : Both myosin of A-band and actin of I-bands are contractile proteins and decrease in size during muscle contraction.

Options

A. If both Assertion and Reason are true and Reason is correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not correct explanation of Assertion.
C. If Assertion is true but Reason is false
D. If both Asssertion and Reason are false.

Answer

To solve the question regarding the assertion and reason about muscle contraction, let's break it down step by step.

### Step 1: Analyze the Assertion
The assertion states: "In muscle contraction, the length of both A-bands and I-bands decrease."
- **Explanation**: During muscle contraction, the I-band (which contains only actin filaments) does indeed shorten as the actin filaments slide over the myosin filaments. However, the A-band (which contains both myosin and overlapping actin filaments) does not change in length; it remains constant. Therefore, the assertion is **false**.

### Step 2: Analyze the Reason
The reason states: "Both myosin of A-band and actin of I-bands are contractile proteins and decrease in size during muscle contraction."
- **Explanation**: While it is true that myosin and actin are contractile proteins, they do not decrease in size during contraction. Instead, the interaction between these proteins causes the muscle fibers to contract, but the lengths of the myosin and actin filaments themselves remain constant. Thus, the reason is also **false**.

### Step 3: Conclusion
Since both the assertion and the reason are false, we can conclude:
- The correct answer is: **Option 4: Both assertion and reason are false.**

### Summary of Steps:
1. Evaluate the assertion about the lengths of A-bands and I-bands during contraction.
2. Evaluate the reason regarding the properties of myosin and actin during contraction.
3. Conclude that both the assertion and reason are false.